Method of preparation of alloys with a base of magnesium-zirconium for improving the mechanical properties of these alloys at high temperatures

ABSTRACT

A method of preparation of alloys with a base of magnesiumzirconium, in which the zirconium is introduced into the alloys in the form of a mother alloy Zr-Mg in the presence of a flux having a high content of potassium salts, especially potassium chloride and/or potassium fluoride, associated if required with other alkali and alkaline-earth salts, the potassium salts representing at least 60 percent and preferably between 70 and 90 percent by weight of the salts introduced into the molten bath, said salts also representing between 7 and 12 percent by weight of the molten metal. A purifying flux is employed representing between 2 and 4 percent by weight of the molten metal, and manganese is introduced into the molten alloy in the form of manganese chloride.

United States Patent 72] Inventors Rene Lucien Neuilly-sur-Seine; Emile Tetart, Basses Pyrenees, both 01' France [21] Appl. No. 740,718

[22] Filed June 27, 1968 Patented Nov. 23, 1971 [73] Assignee Messier Paris, France [32] Priorities July 5, 1967 [3 3 France Mar. 12, 1968, France, No. 143,361

10 Claims, 6 Drawing Figs.

[52] [1.8. CI. 75/168 [51 Int. Cl C22c 23/00 Field oISearch ..75/l68, 135

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney-Waters, Roditi, Schwartz & Nissen ABSTRACT: A method of preparation of alloys with a base of magnesium-zirconium, in which the zirconium is introduced into the alloys in the form of a mother alloy Zr-Mg in the presence of a flux having a high content of potassium salts, especially potassium chloride and/or potassium fluoride, associated if required with other alkali and alkaline-earth salts, the potassium salts representing at least percent and preferably between and percent by weight of the salts introduced into the molten bath, said salts also representing between 7 and 12 percent by weight of the molten metal. A purifying flux is employed representing between 2 and 4 per.- cent by weight of the molten metal, and manganese is introduced into the molten alloy in the form of manganese chloride.

PATENTEDnuvzsusn SHEET 2 BF 6 am? M w m METHOD OF PREPARATION OF ALLOYS WITH A BASE OF MAGNESIUM-ZIRCONIUM FOR IMPROVING THE MECHANICAL PROPERTIES OF THESE ALLOYS AT HIGH TEMPERATURES The present invention relates to alloys with a base of magnesium-zirconium (Mg-Zr) having improved mechanical properties at high temperatures. The invention has also for its object methods of preparation of alloys of this kind.

It is well known that zirconium plays the part of a grainrefming agent of special importance in magnesium alloys; this property is especially described in French Pat. No. 845,843 of Nov. 9, 1948. On the other hand, it is also known that these two materials have excellent nuclear properties.

These two characteristics have made it possible to utilize pure magnesium refined with zirconium as a sheath or can material in atomic piles.

Researches carried out by the applicants on the mechanical characteristics of these alloys at temperatures comprised between 400and 500 C. have shown that the properties in the hot state are improved in cases where the introduction of the zirconium in the binary system Mg-Zr or in more complex alloys such as Mg-Zr-Pb, Mg-Zr-Bi, Mg-Zr,Ce, etc., is made with reducible salts of zirconium.

This introduction of zirconium by means of reducible chemical compounds has already been described in the case of potassium fluozirconate or zirconium fluoride, in particular in British Pat. No. 922,551 of Feb. 19, 1946. In order to reduce the risk of explosion during the reduction of the potassium fluozirconate, numerous authors have furthermore described the utilization of reaction-retarding agents, in particular potassium chloride, which leads to a reaction without explosion (British Pat. No. 947,143 of May 22, I947).

As this preparation implies the use of fluozirconate, the applicants have endeavoured to obtain this improvement of the mechanical properties in the hot state, without at the same time being forced to utilize reducible salts of zirconium, which are frequently more difficult to handle than the mother alloys of Zr-Mg, in which the zirconium is in the metallic state.

Tests carried out by the applicants have made it possible to establish that the introduction of the zirconium in the metallic state (mother alloy) results in properties in the hot state which are substantially identical with those obtained by introduction from fluozirconate, provided that the addition of the metallic Zr-Mg is completed by a treatment with potassium salts, in particular with potassium fluoride or chloride, potassium fluozirconate constituting only one particular case of this family of compounds. 7

However, as the utilization of a flux with a high content of salt does not exclude that of the conventional fluxes for purifying metal, which eliminate the oxide and prevent oxidation of the alloy, the total quantity of salts introduced by the two types of flux is liable to form an inert mass at the bottom of the crucible containing the molten alloy, and this mass adversely affects the operations of preparation and tends to absorb a substantial part of the molten metal,.which considerably increases the permissible losses for a melting operation. In addition, a large mass of salts is liable to contaminate the alloy thus prepared.

Tests carried out by the applicant have proved that it is possible to avoid these serious drawbacks by utilizing one or more fluxes in which the potassium salts represent at least 60 percent by weight and preferably between 70 and 90 percent, of the salts introduced into the bath of molten metal.

In consequence, the invention has for its object a method of preparation of alloys with a base of magnesium-zirconium, according to which the zirconium is introduced into the alloys in the form of a mother alloy of Zr-Mg, in the presence of a flux with a high content of potassium salts, especially of potassium chloride and/or fluoride, associated when so desired with other alkali or alkaline-earth salts, this method being characterized in that the potassium salts represent at least 60 percent and preferably between 70 and 90 percent by weight of the salts introduced into the bath of molten metal.

According to an important characteristic feature of the invention, the potassium salts must preferably represent between 7 and 12 percent of the weight of the molten metal, and the purifying flux should be 2 to 4 percent of the weight of the molten metal.

If the flux of potassium salts comprises difierent salts, for example a mixture of potassium chloride and potassium fluoride, the relative proportions of the different salts may have absolutely any value, on condition that the whole of the potassium salts represents at least 60 percent by weight of the salts introduced into the bath. It is however clear that due to the high cost of potassium fluoride, potassium chloride will preferably be employed.

The advantages offered by the use of a flux with a high content of potassium salts are emphasized for example by the fact that an alloy with a base of magnesium-zirconium has the same characteristics in the hot state, whether it has been treated in a manner known per se with 17.5 kg. of a complex flux containing 57 percent of potassium chloride per kg. of alloy, or, according to the invention, with 10 kg. of potassium chloride and 2.5 kg. of flux per 100 kg. of alloy.

The detailed description given below will show that the association of potassium chloride and fluoride confers, at the same time as satisfactory melting points, greatly improved properties in the hot state, while limiting the quantities of chloride to be employed, and which would be liable to contaminate the final product of melting.

In this description, reference will be made to the accompanying drawings in which:

FIG. 1 is a diagram illustrating the elongation as a function of time of Mg-Zr alloys which have been subjected to various treatments.

FIG. 2 is a diagram illustrating the breaking stress as a function of time for alloys of Mg-Zr having been subjected to different treatments;

FIGS. 3, 4, 5 and 6 illustrate the elongation as a function of time of various alloys with a base of Mg-Zr.

The series of curves of FIG. 1 shows the elongation obtained at 280 C., at the end of a period of time indicated in abscissae and under a load of 3.4 h-bars, for test samples of binary alloys of MgZr, with the following methods of preparation:

Mg-Zr (0.40-0.60 percent Zr):

Preparation by mother alloy without special treatment with potassium salts, with the exception of potassium chloride KCl contained in the purifying flux;

Mg-Zr (0.40-0.60 percent Zr):

Preparation including the use of potassium fluozirconate;

Mg-Zr (0.400.60 percent Zr):

Preparation by mother alloy without special treatment with potassium salts, with the exception of potassium chloride KC] contained in the purifying flux;

Mg-Zr (0.400.60 percent Zr):

Preparation including the use of potassium fluozirconate;

Mg-Zr (0.400.60 percent Zr):

Preparation by mother alloy and potassium salts for various quantities expressed in percent of the weight of metal treated in the crucible.

The following properties are noted:

1. Preparation by means of fluozirconate considerably reduces the elongation obtained for a given time, that is to say reduces the speed of creep under these conditions:

2. Preparation from the mother alloy Zr-Mg, completed by treatment with potassium salts at large contents, makes it possible to obtain speeds of creep less than those obtained from fluozirconate. It is observed that the action of the potassium salts increases with the quantities of salts introduced into the field described.

The system of curves of F IG. 2 illustrates the breaking stress as a function of time for test samples of Mg-Zr (0.400.60 percent Zr) prepared by:

Mother alloy Zr-Mg without special treatment with potassium salts;

Mother alloy Zr-Mg, with special treatment with potassium salts;

Potassium fluozirconate.

It will be noted that under a stress of 3.4 h-bars, fracture takes place at 280 C. after:

7 minutes in the first case;

37 minutes in the second case;

17 minutes in the third case.

The improvement in behavior under heat due to the preparation by mother alloy Zr-Mg, accompanied by a treatment with potassium salts is thus particularly marked.

This effect is again found in the temperature range of 400 to 500 C., which is that of the operation of nuclear generators. It is observed in this field that the elastic limit under tension is also improved in a very substantial manner by preparation with the mother alloy Zr-Mg, completed by a treatment with potassium salts.

An example of application is given below in the case of the mixture of KF and KCI containing 20 percent of KF. This mixture is previously prepared by the introduction of KF into molten KC]. The product is then poured into an ingot mould and subsequently ground.

The treatment by potassium salts is effected for example at 780 C. and produces the results indicated.

The curves of FIG. 3 show the influence of alkaline-earth elements and indicate the results obtained with equal quantities of salts of:

Potassium-(K) Magnesium-(Mg) Calcium-(Ca) Barium-(Ba).

The conditions of the tests (280 C., 3.4 h-bars) are the same as before.

It will be noted that the action of the alkaline earth elements on the properties in the hot state is only slight as compared with that of potassium.

Even very small additions of manganese to the Mg-Zr alloy considerably increase the behavior under heat of the alloy treated with potassium salts, their action on this alloy not treated with potassium salts being much weaker, as is shown by the curves of FIG. 4 (under the same test conditions as before). It results from the work of the applicants that the contents of manganese which produce the best characteristics under heat are comprised between 0.2 and 0.3 percent by weight. Tests carried out by the applicants have proved that the manganese should preferably be introduced into the bath in the form of manganese chloride.

The preparation forming the object of the invention, which improves the mechanical properties while hot of the binary system Mg-Zr brings the same advantages to complex, ternary, quaternary alloys, etc., with a base of Mg-Zr and results in new products, new in that they are different in their properties from alloys of the same basic composition obtained by conventional preparation with the mother alloys Zr-Mg. I

The systems of curves of FIGS. 5 and 6 show the elongations obtained at 280 C. under 3.4 h-bars, as a function oftime and for ternary or quaternary alloys prepared from the mother alloy Zr-mg, with or without additional treatment with potassium salts.

In all cases, there is observed an improvement in the mechanical characteristics while hot caused by this preparation.

The alloys Mg-Zr-Pb-Bi have properties under heat which are particularly improved by a preparation of this kind, and this characteristic, together with their excellent nuclear properties, lends itself to important applications in nuclear generators.

The addition of to 0.3 percent of manganese to these alloys results in a further improvement in their properties at high temperatures.

The curves shown in FIGS. and 6 correspond to the following preparations:

0.7 percent of Pb:

a. Mother alloy Zr-Mg; b. Mother alloy Zr-Mg treatment with potassium salts.

1.4 percent of Pb: do.

4 Mg-Zr-Pb-Bi l.4 percent Pb, 0.5 percent Bi:

a. Mother alloy Zr-Mg; b. Mother alloy Zr-Mg treatment with potassium salts.

1.4 percent Pb, 1 percent Bi: do.

The elastic limits within the temperature range of 400500 C. are obtained by rapid tensile strength tests at the temperatures indicated.

The variations of elastic limits are classified as variations of behavior to rapid creep expressed previously (A%=- f(time)).

This preparation is applicable to all alloys containing elements compatible with zirconium, that is to say with the special exclusion of A1, Si, Fe, etc. In particular, the following elements may be present in the alloys, in addition to those already indicated in the previous description: Ag, Cd, Ta, Ca, Ga, Sn, Mn, Li, Cu, Th, Zn, etc.

In the preparation of these alloys, the introduction of the zirconium in the form of the mother alloy Zr-Mg and potassium salts is effected for example as previously described.

The new properties obtained by the preparation which has been described above permit the utilization of these complex alloys with a base of Mg-Zr in mechanical construction work, and especially in aeronautics, but they have a special advantage in the case of sheathing materials of atomic piles, since they render possible the use of alloys containing elements having very special nuclear properties such as Pb, Bi, for example; it was not possible to adopt these alloys until the present time for such applications, because of their inadequate characteristics under heat.

What I claim is:

1. A method of preparation of alloys with a base of magnesium-zirconium, said method consisting of introducing into a molten metal bath with a base of magnesium a mother alloy of Zr-Mg in the presence of a flux having a high content of potassium salts, said potassium salts constituting at least 60 percent by weight of the salts introduced into the molten bath.

2. A method of preparation of alloys with a base of magnesium-zirconium, said method consisting of introducing a mother alloy of Zr-Mg into a molten metal bath with a base of magnesium in the presence of a flux having a high content of potassium salts, said potassium salts constituting at least 60 percent by weight of the salts introduced into said molten bath, and between 7 and 12 percent by weight of the molten metal.

3. A method of preparation of alloys with a base of magnesium-zirconium, said method consisting of introducing a mother alloy of Zr-Mg into a molten metal bath with a base of magnesium, in the presence of a flux having a high content of potassium salts, said potassium salts constituting between 70 and percent by weight of the salts introduced into said molten bath.

4. A method of preparation of alloys with a base of magnesium-zirconium, said method consisting of introducing a mother alloy of Zr-Mg into a molten metal bath with a base of magnesium in the presence of a flux having a high content of potassium salts, said potassium salts constituting between 70 and 90 percent by weight of the salts introduced into said molten bath, and between 7 and 12 percent by weight of said molten metal.

5. A method as claimed in claim 1, wherein there is introduced into said molten bath, a purifying flux constituting between 2 and 4 percent by weight of the molten metal.

6. A method as claimed in claim 3, wherein there is introduced into said molten bath, a purifying flux constituting between 2 and 4 percent by weight of the molten metal.

7. A method as claimed in claim 1, wherein manganese is introduced into said molten bath, in the form of manganese chloride.

8. A method as claimed in claim 7, wherein said manganese constitutes between 0.2 and 0.3 percent by weight of the metals contained in said molten bath.

9. A method as claimed in claim I, wherein the potassium salt is a potassium halide.

10. A method as claimed in claim 9, wherein the potassium halide is potassium chloride or potassium fluoride. 

2. A method of preparation of alloys with a base of magnesium-zirconium, said method consisting of introducing a mother alloy of Zr-Mg into a molten metal bath with a base of magnesium in the presence of a flux having a high content of potassium salts, said potassium salts constituting at least 60 percent by weight of the salts introduced into said molten bath, and between 7 and 12 percent by weight of the molten metal.
 3. A method of preparation of alloys with a base of magnesium-zirconium, said method consisting of introducing a mother alloy of Zr-Mg into a molten metal bath with a base of magnesium, in the presence of a flux having a high content of potassium salts, said potassium salts constituting between 70 and 90 percent by weight of the salts introduced into said molten bath.
 4. A method of preparation of alloys with a base of magnesium-zirconium, said method consisting of introducing a mother alloy of Zr-Mg into a molten metal bath with a base of magnesium in the presence of a flux having a high content of potassium salts, said potassium salts constituting between 70 and 90 percent by weight of the salts introduced into said molten bath, and between 7 and 12 percent by weight of said molten metal.
 5. A method as claimed in claim 1, wherein there is introduced into said molten bath, a purifying flux constituting between 2 and 4 percent by weight of the molten metal.
 6. A method as claimed in claim 3, wherein there is introduced into said molten bath, a purifying flux constituting between 2 and 4 percent by weight of the molten metal.
 7. A method as claimed in claim 1, wherein manganese is introduced into said molten bath, in the form of manganese chloride.
 8. A method as claimed in claim 7, wherein said manganese constitutes between 0.2 and 0.3 percent by weight of the metals contained in said molten bath.
 9. A method as claimed in claim 1, wherein the potassium salt is a potassium halide.
 10. A method as claimed in claim 9, wherein the potassium halide is potassium chloride or potassium fluoride. 